The LASCO Coronal Brightness Index

Abstract

We present the construction of a new white-light coronal brightness index (CBI) from the entire archive of observations recorded by the Large Angle Spectrometric Coronagraph (LASCO) C2 camera between 1996 and 2017, comprising two full solar cycles. We reduce all fully calibrated daily C2 observations of the white-light corona into a single daily coronal brightness observation for every day of observation recorded by the instrument, with mean daily brightness values binned into 0.1 $\mathrm{R}_{\odot}$ radial $\times1$ degree angular regions from 2.4 – 6.2 $\mathrm{R}_{\odot}$ for a full 360 degrees. As a demonstration of the utility of the CBI, we construct a new solar irradiance proxy that correlates well with a variety of direct solar irradiance observations, with correlations shown to be in the range of 0.77 – 0.89. We also present a correlation mapping technique to show how irradiance correlations depend on, and relate to, coronal structure/locations, and to demonstrate how the LASCO CBI can be used to perform long-term “spatial correlation” studies to investigate relationships between the solar corona and any arbitrary concurrent geophysical index. Using this technique we find possible relationships between coronal brightness and plasma temperature, interplanetary magnetic field magnitude and (very weakly) proton density.

Appendix: Data Sharing: Obtaining the LASCO CBI

We are making available the LASCO CBI as both raw and interpolated data sets, along with the relevant time information, in a number of formats. The primary CBI data product is a $360\times38\times7094$ data cube representing values obtained from existing data files only (i.e., the time series contains many discontinuities). An alternative data cube we provide is a $360\times38 \times7777$ data cube representing interpolated observations (i.e. linear interpolation across all missing dates). The CBI data cubes are available as both IDL “.sav” save files and Python/NumPy “.npy” files. File sizes are of the order 406 megabytes for the interpolated data set and 370 megabytes for the raw (not interpolated) data.

Additionally, we provide both “raw” and “full” data files containing the relevant date information (YYYY-M-D) in the following formats: i) IDL ‘.sav’ files; ii) plain text files; iii) NumPy .npy files containing datetime objects for the dates. An accompanying “README” file will contain this information and identify which data products correspond to which filenames. This README file will be updated as the CBI is updated to include new observations. We intend to support and maintain this product through the LASCO mission lifetime.

All aforementioned files are available at the following url: https://lasco-www.nrl.navy.mil/CBI. Should investigators require any additional metadata, we encourage them to contact the corresponding author of this article. We also request that any publications utilizing the LASCO CBI data cite this publication.